This invention relates to fluid temperature switches, and more particularly to the fitting of such a switch into the wall of a chamber containing a fluid and most notably into the wall of an automobile radiator header tank.
A fluid temperature switch unit takes the form of a generally cylindrical insulating enclosure, one end of which contains a bimetal disc which deforms under heat whereby to make or break contact between two electrical conductors connected to terminal tags projecting from the enclosure. Such switches are made wide use of in the header tanks of engine cooling radiators on automobiles, for instance for stopping or starting -- depending on the water temperature -- a fan for accelerating the airflow through the radiator.
The walls of header tanks are ordinarily made of metal and, in order to mount a water temperature switch thereon, the latter is set into a threaded brass plug which is screwed into a hole tapped in the wall in such manner that the metal of the plug bear leaktightly against the metal of the wall. It is necessary for such plug to possess a high calorific capacity so that it should not assume (nor transmit to the bimetal disc of the switch) the mean temperature prevailing in the metal wall of the header tank as a result of the heat transfer taking place between the wall and the exterior. Because of its high calorific capacity, the plug will follow the temperature fluctuations of the water inside the header tank with a certain time-lag, but nevertheless more faithfully than if it had low calorific capacity. The threaded plug must therefore have a large mass of brass and is relatively very costly, and in fact the direct cost of a switch set into such a threaded plug is about twice as much as that of the switch alone.
Use is also made of header tanks having walls made of synthetic resin. In order to mount a water temperature switch into such walls, the current technique is to screw the threaded brass plug containing the switch into a tapped brass sleeve which engages into a recess in the wall. However, it is almost impossible to obtain satisfactory sealing between the brass sleeve and the recess in synthetic resin. The plug must therefore be sealingly applied against a portion of the synthetic resin recess that projects beyond the brass sleeve, thereby complicating the fitting operation.